As described in JP-A No. 236634/1991, conventional FM transmitters for transmitting FM modulation signals use phase lock loops (PLL).
Typically, such an FM transmitter is composed of a reference oscillator, a phase comparator, a programmable frequency divider, a low pass filter (L.P.F), a sample-and-hold circuit, an adder, a voltage controlled oscillator (VCO), a buffer amplifier (AMP), and a transmission antenna. Operation of such a typical FM transmitter is described below.
The output of the voltage controlled oscillator is divided by the programmable frequency divider and entered to one input terminal of the phase comparator. A reference signal is then supplied to the other input terminal of the phase comparator from the reference oscillator. The phase comparator outputs a signal having a pulse width equivalent to a phase difference between the above two signals. The output signal of the phase comparator is fed back to the frequency control terminal of the voltage controlled oscillator via the low pass filter, the sample-and-hold circuit, and the adder. The PLL is composed of a series of the connections among the devices. Due to the feedback configuration, the frequency of the signal output from the voltage controlled oscillator becomes a frequency obtained by multiplying the reference signal by the frequency division number of the programmable frequency divider.
The conventional FM transmitter is also provided with a microcomputer that receives signals from the phase comparator and the programmable frequency divider. The microcomputer detects a locked PLL with a signal received from the phase comparator. The microcomputer then starts up the sample-and-hold circuit according to the detection of the locked PLL and holds as a sample the control voltage of the voltage controlled oscillator used while the PLL is locked. After that, the frequency control signal obtained from transmit data is added up by the adder via a digital/analog converter and a low pass filter, whereby an FM modulation signal is obtained. The low pass filter, used to convert the output of the phase comparator to DC signals required for the voltage controlled oscillator in the PLL, is referred to as a loop filter. In addition, the FM modulation signal generated from the voltage controlled oscillator is supplied to the antenna via the buffer amplifier, whereby FM modulation signals are transmitted.
The conventional FM transmitter uses a sample-and-hold circuit so as to cut the closed PLL and moves the PLL into an open loop control state. The adder is used to add the sample-held output voltage of the loop filter to a signal that causes a frequency offset corresponding to a transmission signal, thereby generating an FM modulation signal. At this time, because the sample-and-hold circuit is controlled according to a lock signal output from the phase comparator and detected by the microcomputer, the FM transmitter can be moved into a state in which an FM modulation signal is generated automatically when the PLL is locked.
In the conventional FM transmitter, however, no consideration is given to the controlling of the start-up of the buffer amplifier, resulting in the FM transmitter being high in power consumption. Likewise, the conventional FM transmitter, because of its requirement of a sample-and-hold circuit to move the PLL into open loop control, causes the power consumption to increase.